This invention relates to data processing systems, and in particular to data processing systems that include diagnostic apparatus. The purpose of diagnostic apparatus is to monitor the operation of the data processing system, checking that it is functioning correctly, and, if it is not, providing information about the nature of the fault.
In carrying out this function the diagnostic apparatus is usually connected to receive signals indicating the current state of parts of the data processing system.
This state may be monitored to ensure that it is correct. In addition it has been proposed to make the diagnostic apparatus capable of pre-empting control of the data processing system to cause it to carry out some operations determined by the diagnostic apparatus. The course or results of these operations is then monitored.